The present invention relates to a lithium secondary battery which maintains a good charge-discharge characteristic even during a high output cycle operation and which in particular may be preferably used for a drive motor of an electric vehicle.
In recent years, it is eagerly desired to regulate the emissions of carbon dioxide with the environment protection movement. In the automobile industry, to replace automobiles using fossil fuels, such as vehicles driven by gasoline, there is a movement to promote the introduction of electric vehicles (EV) and hybrid electric vehicles (HEV) A lithium secondary battery acting as a motor-drive battery acting as a key for putting EV and HEV into practical use is required to have not only a huge battery capacity but also a huge battery output, which greatly affects the acceleration performance as well as the slope-climbing performance of the vehicle.
In general, the internal electrode body of a lithium secondary battery has a positive electrode, a negative electrode and a separator made of porous polymer film, the positive electrode and the negative electrode being wound or laminated so that the positive electrode and negative electrode are not brought into direct contact with each other via the separator. For example, as shown in FIG. 1, an internal electrode body 1 of a winding type is formed by winding a positive plate 2 and a negative plate 3 having a separator 4 in between, and a tab 5 is provided for each of the positive and negative electrode plates 2, 3 (hereafter referred to as xe2x80x9celectrodes 2, 3xe2x80x9d) respectively. The end opposite to the end connected to the electrodes 2, 3 of each tab 5, is attached to an external terminal 11 or an electric current extracting terminal 13 such as an internal terminal member 12 being electrically connected to the external terminal 11. That is, the tab 5 serves as a lead line which is electrically connected to the electric current extracting terminal 13 while conducting electricity collection from the electrodes 2, 3.
Here, a plan view of the electrodes 2, 3 when the internal electrode body 1 is spread out is shown in FIG. 2. The electrodes 2, 3 are formed with an electrode active material 16 coated respectively onto metal foils 15 made of aluminum, etc., for the positive electrode 2 and made of copper, etc., for the negative electrode 3. Since a tab 5 is provided on a side of such metal foil 15, a tab having a thin band shape is preferably used. The tabs are disposed at approximately a uniform distance so that each tab 5 conducts electricity collection in a constant area in electrode 2, 3. Incidentally, in general, the material qualities of the tabs 5 are the same as the material qualities of the metal foil 15 to which the tabs 5 are attached.
With respect to a lithium secondary battery for EV or HEV, there are cases in which a current equal to or more than 100 A flows per battery. In the case where such a huge current flows, there is a need for the internal resistance of all the batteries to be made as low as possible in order to reduce the output loss of the batteries.
Therefore, it goes without saying that it is preferred to make the resistance of the internal electrode body low, but now, paying attention to the connection path from the formerly-described internal electrode body 1 to an electric current extracting terminal 13, it is preferable that the resistance of the members of metal foil 15, tab 5 and electric current extracting terminal 13 be low. However, concerning the metal foil 15 and electric current extracting terminal 13, there is a certain limit to making the resistance value lower due to the fact that the material is limited as well as due to limitations from the point of view of the shape of the battery and energy density.
On the other hand, the tab 5 has an allowable range in which to set a resistance value from the point of view of the feasibility of setting its shape freely since the shape of tab 5 is to be housed in the space between the battery case housing the internal electrode body 1 therein and the internal electrode body 1. In addition, concerning the tabs 5 and the metal foil 15, the connection resistance of these does not vary much, since they are unified by welding, except for with extremely faulty welding. Concerning the connection between the tabs 5 and the electric current extracting terminal 13, however, there is room left to reduce the contact resistance, since various methods may be considered. For example, for tabs with the shape of a thin band, a method of bundling by piling up in one direction is preferably easiest in terms of a forming process for the battery, and also is preferred since the structure inside the battery will not become complicated. In this case, however, there will be a need to aim to reduce the contact resistance on the contact surface of each tab, since contact between tabs will occur more times.
Concerning the tabs 5 and the method used to connect the tab 5 to the electric current extracting terminal 13, however, the resistance of these members themselves and the connection resistance, which did not occupy a larger percentage from the view point of internal resistance of the entire battery, were not considered to be important, and how the dispersion (distribution) of this resistance may affect the output characteristic or charge-discharge cycle operation characteristic has not been explained.
The present invention was achieved by considering the problems of the prior art mentioned above. That is, according to the present invention, there is provided a lithium secondary battery, comprising a battery case, an internal electrode body contained in the battery case and including a positive electrode, a negative electrode and a separator made of porous polymer, the positive electrode and the negative electrode being wound or laminated so that the positive electrode and negative electrode are not brought into direct contact with each other via the separator, and at least plural tabs, having been connected to the positive electrode and negative electrode for electricity collection so that the respective resistance value of each tab remains within the range of xc2x120% of the average resistance value of the tabs.
In a lithium secondary battery of the present invention, it is preferred that all the tabs are concentrated at one place and are connected to the electric current extracting terminal by crimping or welding. In addition, it is also preferred that all the tabs are connected to the electric current extracting terminal by crimping, or welding, or screwing after they have been unified by crimping, welding, or eyelet-type connecting in advance. Incidentally, the thickness of a tab is preferably 5 xcexcm or more and 100 xcexcm or less, and the battery structure using such tabs is preferably adopted to a lithium secondary battery with a battery capacity of 5 Ah or more, and especially to a lithium secondary battery for an EV or a hybrid electric vehicle.